The Vertical Cavity Surface Emitting Laser (VCSEL) is rapidly becoming a workhorse technology for semiconductor optoelectronics. VCSELs can typically be used as light emission sources anywhere other laser sources (e.g., edge emitting lasers) are used, and provide a number of advantages to system designers. Hence, VCSELs are emerging as the light source of choice for modern high-speed, short-wavelength communication systems and other high-volume applications such as optical encoders, reflective/transmissive sensors and optical read/write applications.
Surface-emitting lasers emit radiation perpendicular to the semiconductor substrate plane, from the top or bottom of the die. A VCSEL is a surface-emitting laser having mirrors disposed parallel to the wafer surfaces that form and enclose an optical cavity between them. VCSELs usually have a substrate upon which a first mirror stack and second mirror stack are disposed, with a quantum well active region therebetween. Gain per pass is much lower with a VCSEL than an edge-emitting laser, which necessitates better mirror reflectivity. For this reason, the mirror stacks in a VCSEL typically comprise a plurality of Distributed Bragg Reflector (DBR) mirrors, which may have a reflectivity of 99% or higher. An electrical contact is usually positioned on the second mirror stack, and another contact is provided at the opposite end in contact with the substrate. When an electrical current is induced to flow between the two contacts, lasing is induced from the active region and emits through either the top or bottom surface of the VCSEL.
VCSELs may be broadly categorized into multi-transverse mode and single-transverse mode, each category being advantageous in different circumstances. A goal in manufacturing single-mode VCSELs is to assume single-mode behavior over all operating conditions, without compromising other performance characteristics. Generally, the active regions of single transverse mode VCSELs require small lateral dimensions, which tend to increase the series resistance and beam divergence angle. Furthermore, a device that is single-mode at one operating condition can become multi-mode at another operating condition, an effect that dramatically increases the spectral width and the beam divergence of the emitted radiation of the VCSEL.
Depending upon the application, the output mode of a VCSEL can either positively or negatively affect its use in signal transmission and other applications. The mode structure is important because different modes can couple differently to a transmission medium (e.g., optical fiber). Additionally, different modes may have different threshold currents, and can also exhibit different rise and fall times. Variation in threshold currents, which can be caused by different modes, combined with different coupling efficiencies of different modes can cause coupling into a transmission medium to vary in a highly non-linear manner with respect to current. Variable coupling to a transmission medium, combined with different rise and fall times of the various modes, can cause signal pulse shapes to vary depending on particular characteristics of the coupling. This can present problems in signal communications applications where transmission depends on a consistent and reliable signal. Other applications (e.g., printing devices, analytical equipment) may require a consistent and focused light source or spectral purity characteristics that render multiple mode sources inefficient or unusable.
Manufacturing a VCSEL with mode control and high performance characteristics poses a number of challenges. It is difficult to manufacture VCSELs that efficiently operate in the lower order mode (single mode). Most conventional VCSELs tend to lase in higher-order transverse modes, whereas single transverse mode lasing is preferred for some applications, such as sensors. Conventional attempts to produce a single mode VCSEL have generally resulted in structures having output power insufficient for practical use in most applications, as they remain single mode only over small current ranges. Usually, to manufacture a VCSEL, a relatively large current aperture size is required to achieve a low series resistance and high power output. A problem with a large current aperture is that higher order lasing modes are introduced so that single mode lasing only occurs just above threshold, if at all. Manufacturing a VCSEL with a smaller current aperture to obtain single mode behavior causes multiple problems: the series resistance becomes large, the beam divergence angle becomes large, and the attainable power becomes small. Some conventional anti-guide structures may achieve this but suffer from manufacturing difficulties, particularly in requiring an interruption in epitaxial growth, a patterning step, and subsequent additional epitaxy. Other large single mode VCSELs require multi-step MBE or MBE/MOCVD combinations to manufacture, creating alignment and yield problems; increasing production costs and reducing commercial viability.